Study of D0– mixing and D0 doubly Cabibbo-suppressed decays

Using a sample of four million hadronic Z events collected in ALEPH from 1991 to 1995, the decays D*+-->D0πs+, with D0 decaying to K-π+ or to K+π-, are studied. The relative branching ratio B(D0-->K+π-)/B(D0-- >K-π+) is measured to be (1.84+/-0.59(stat.)) (+/-0.34(syst.))%. The two possible contributions to the D0-->K+π- decay, doubly Cabibbo-suppressed decays and D0-D ̄0 mixing, are disentangled by measuring the proper-time distribution of the reconstructed D0's. Assuming no interference between the two processes, the upper limit obtained on the mixing rate is 0.92% at 95% CL. The possible effect of interference between the two amplitudes is also assessed.